1. Field of the Invention
The present invention generally relates to a system for transmitting control signals and supervisory signals. More specifically, the invention is concerned with a bidirectional control/supervisory signal transmission system which includes a central station and at least a local station which is located remotely from the former and connected thereto by way of a common data signal transmission line and which includes at least an instrument, device or machine to be controlled and monitored (hereinafter also referred to collectively as the controlled devices), wherein control signals generated in parallel in the central station undergo a parallel-to-serial conversion to be transmitted serially to the local station for controlling in parallel the controlled devices after having undergone a serial-to-parallel conversion, while in the local station, supervisory signals indicating the various operating states of the controlled devices are generated and subjected to a parallel-to-serial conversion to be serially transmitted to the central station by way of the common data signal transmission line. In the central control station, the serial supervisory signals are converted into parallel signals for evaluation.
2. Description of the Prior Art
In the field of the automatic control techniques, there is employed widely such a signal transmission system in which control signals are transmitted from a control station including a sequence controller, programmable controller, a computer or the like are transmitted to a number of controlled devices such as electric motors, solenoids, electromagnetic valves, relays, thyristors, lamps and/or the like installed at locations remotely from the control station for controlling the operation thereof, and in which the supervisory signals indicating the states of the controlled devices detected by sensor means such as reed switches, micro-switches, push-button switches or the like bi-state (on/off-state) sensor elements are transmitted to the control station for the evaluation of the operating states of the devices.
The prior art control/supervisory signal transmission system of the type mentioned above requires a number of transmission lines such as power transmission lines, control/supervisory signal transmission lines, clock signal lines, ground potential (reference) lines and others to be connected between the control station and the controlled devices as well as between the control stations and the individual sensors, which involves necessarily a very large amount of hardware for wiring the transmission lines and high expenditure or cost. Besides, in the present state of the art, the controlled devices tend to be implemented in more and more miniaturized size and disposed with a high density, as a result of which difficulty is encountered in making access to these devices because of limitation in the available space, rendering the wiring more complicate and troublesome.
As a typical example of the control/supervisory signal transmission system known heretofore, there can be mentioned such a system in which the control station and the controlled devices are interconnected by a power transmission line adapted to transmit electric power to the controlled devices with data or information signals being superposed thereon and a ground potential line. Reference may be made to, for example, Japanese Patent Publication No. 4239/1986. This transmission system however suffers from problems in that a lot of time is taken for the transmission or transfer of data or information signal between the control station and a plurality of the controlled devices and that a mechanism for detecting addresses is required because address information, each consisting of plural bits, is contained in the information signals as transferred.